Basic HRCT Patterns: Ground-glass
Opacity

Definition: Ground-glass
refers to the HRCT appearance of a hazy opacity that does not obscure
the associated pulmonary vessels. This appearance results from
parenchymal abnormalities that are below the spatial resolution of
HRCT. Ground-glass opacity can be seen with alveolar wall
inflammation or thickening, with partial air-space filling, or with
some combination of the two.

How to Recognize
Ground-glass Opacity

This image shows patchy ground-glass
opacities throughout both lungs. Note that in the regions of
ground-glass, one can see the vessels.

This patient had influenzal
pneumonia.

Common causes (some of which are not routinely imaged
by HRCT) include pulmonary edema; ARDS; viral, mycoplasmal, and
pneumocystis pneumonias; hypersensitivity pneumonia; pulmonary
hemorrhage; and other diffuse interstitial lung diseases.

Many air-space filling processes can also result in
consolidation in advanced disease.

HRCT after
Recovery

A scan performed after recovery from the
pneumonia shows a marked decrease in ground-glass opacity.
This image is taken at the same level as that shown above.

Some of the posterior increased density
represents normal dependent lung.

Gross Appearance

This slice of lung from a patient who died
of influenza pneumonia was heavier and redder than normal
and uniformly firm throughout. Large airways were
patent.

Although the appearance is often uniform at
postmortem examination, diseases that have ground-glass
opacity on HRCT begin as patchy opacities that become
confluent. The terminal appearance resembles the red
hepatization of bacterial lobar pneumonia, but the
histologic picture differs from the neutrophilic alveolar
filling of bacterial pneumonia.

Histologic
Findings in Influenza Pneumonia

Alveolar walls are thickened by edema
and RBCs. Alveolar epithelial cells degenerate, and coagulated
proteinaceous exudate produces hyaline membranes. With time the
membranes are replaced by hyperplastic type II cells, which then
differentiate into type I cells when resolution occurs. Alveolar
spaces contain a proteinaceous exudate and a few inflammatory cells.
The alveolar wall thickening and alveolar exudate still mixed with
air cause the ground-glass opacity on HRCT. Loss of air produces
consolidation.